1. Field of the Invention
The present invention relates to a surface acoustic wave filter used as, for example, a band pass filter in a portable telephone, and more particularly, to a surface acoustic wave filter having a balanced-to-unbalanced conversion function, namely, a balun function.
2. Description of the Related Art
With the reduction in size and weight of portable telephones in recent years, the number and size of components included in portable telephones have been greatly reduced. In addition, components combining a plurality of functions are being developed.
Accordingly, surface acoustic filters used for the RF(radio frequency) stage of portable telephones, having a balanced-to-unbalanced conversion function, namely, a balun function, have been developed, and are primarily used in GSM portable telephones.
FIG. 11 is a schematic plan view showing the electrode structure of a conventional surface acoustic wave filter 100 having a balanced-to-unbalanced conversion function.
In the surface acoustic wave filter 100, first to third IDTs (interdigital transducers) 101 to 103 are arranged on a piezoelectric substrate (not shown) along the propagation direction of a surface acoustic wave. Reflectors 104 and 105 are provided on the opposite sides of the IDTs 101 to 103 in the direction of a surface acoustic wave.
One end of each of the IDTs 101 and 103 is electrically connected to an unbalanced signal terminal 108, and the other end of each IDT 101 and 103 is connected to the ground potential. One end of the IDT 102 is electrically connected to a first balanced signal terminal 106, and the other end thereof is electrically connected to a second balanced signal terminal 107.
The magnitude of the attenuation outside the pass band of a surface acoustic wave filter having a balanced-to-unbalanced conversion function depends upon the degree of balance of the surface acoustic wave filter. The degree of balance is represented as the difference between the transmission characteristic between the unbalanced signal terminal and the first balanced signal terminal, and the transmission characteristic between the unbalanced signal terminal and the second balanced signal terminal. Of these differences in the transmission characteristic, the difference in the amplitude characteristic is called xe2x80x9camplitude balance degreexe2x80x9d and the difference in the phase characteristic is called xe2x80x9cphase balance degreexe2x80x9d.
When a surface acoustic filter having a balanced-to-unbalanced conversion function is defined by a device having first to third ports, and, for example, the unbalanced input terminal thereof is the first port, and the first and second balanced output terminals thereof are the second and third ports, respectively, the amplitude balance degree and the phase balance degree are represented by the following expressions:
Amplitude balance degree=|A|, A=|20xc2x7log(S21)|xe2x88x92|20xc2x7log(S31)|, and 
Phase balance degree=|Bxe2x88x92180|, B=|∠S21xe2x88x92∠S31|
Here, S21 denotes the transmission coefficient from the first port to the second port, and S31 denotes the transmission coefficient from the first port to the third port.
Optimal values of the amplitude-balance degree and the phase balance degree in the outside of the pass band of the filter are 0 dB for the amplitude balance degree, and 0xc2x0 for the phase balance degree. The magnitude of the attenuation outside the pass band in the filter having these optimal degrees of balance is infinity. Therefore, as the amplitude balance degree and the phase balance degree approach 0 dB and 0 degrees, respectively, the attenuation outside the pass band is greatly increased.
In the surface acoustic wave filter 100 shown in FIG. 11, although a balanced-to-unbalanced conversion function can be achieved, the degree of balance is unsatisfactory, and thus, the attenuation outside the pass band is insufficient.
To overcome the above-described problems and drawbacks, preferred embodiments of the present invention provide a longitudinally-coupled resonator-type surface acoustic wave filter that improves the degree of balance outside the pass band in addition to having a balanced-to-unbalanced conversion function, and that greatly increases the attenuation outside the pass band.
A surface acoustic wave filter according to a first preferred embodiment of the present invention includes a piezoelectric substrate, a plurality of IDTs provided on the piezoelectric substrate and arranged along the propagation direction of a surface acoustic wave, and a balanced-to-unbalanced conversion function. Among the plurality of IDTs, the IDTs on opposite sides are disposed in an approximate point-symmetry arrangement relative to the IDT positioned at the approximate center in the propagation direction of a surface acoustic wave.
A surface acoustic wave filter according to a second preferred embodiment of the present invention includes a piezoelectric substrate, first to third IDTs provided on the piezoelectric substrate, and sequentially arranged along the propagation direction of a surface acoustic wave, an unbalanced signal terminal connected to the first and third IDTs, and first and second balanced signal terminals each connected to the opposite ends of the second IDT. In this surface acoustic wave filter, the first to third IDTs have first and second end portions located on opposite sides thereof in a direction that is substantially perpendicular to the propagation direction of a surface acoustic wave, the first end portion of the first IDT and the second end portion of the third IDT are each electrically connected to the unbalanced signal terminal, and the second end portion of the first IDT and the first end portion of the third IDT are each connected to the ground potential.
A surface acoustic wave filter according to a third preferred embodiment of the present invention includes a piezoelectric substrate, first to third IDTs provided on the piezoelectric substrate and sequentially arranged along the propagation direction of a surface acoustic wave, an unbalanced signal terminal connected to the second IDT, and first and second balanced signal terminals connected to the first and third IDT. In this surface acoustic wave filter, the first to third IDTs have first and second end portions located on opposite sides thereof in a direction that is substantially perpendicular to the propagation direction of a surface acoustic wave, the first end portion of the first IDT and the second end portion of the third IDT are each electrically connected to a first balanced signal terminal, and the second end portion of the first IDT and the first end portion of the third IDT are each electrically connected to a second balanced signal terminal.
In the above-described conventional surface acoustic wave filter 100, since the degree of balance outside the pass band is unsatisfactory, the attenuation outside the pass band is insufficient. The reason for this is as follows. Since the balanced signal terminal 106 is surrounded by the signal line connected to the unbalanced signal terminal 108, a parasitic capacitance inserted between input and output terminals in parallel has a large influence, and, since the balanced signal terminal 107 is adjacent to the wiring connected to the ground terminal, a parasitic capacitance added between the signal line and the ground line in parallel has a large influence. That is, the parasitic capacitances added to the balanced signal terminals 106 and 107 differ from each other, and thereby the degree of balance outside the pass band is unsatisfactory.
Due to the above-described problems, the present inventor found that, if an electrode is provided such that substantially equal parasitic impedances are each added to a pair of balanced signal terminals, the degree of balance outside the pass band is greatly improved, and the attenuation outside the pass band will greatly increase. On the basis of this discovery, the present invention was developed.
Accordingly, the surface acoustic wave filter in accordance with preferred embodiments of the present invention includes an electrode structure such that substantially equal parasitic impedances are each added to the first and second balanced signal terminals.
More specifically, in the first preferred embodiment of the present invention, among the plurality of IDTs, the opposite IDTs are disposed in approximate point-symmetry about the IDT positioned at the approximate center in the propagation direction of a surface acoustic wave, and thereby, the substantially equal parasitic impedances are added to the first and second balanced signal terminals.
In the second preferred embodiment of the present invention, the first end portion of the first IDT and the second end portion of the third IDT are each connected to the unbalanced signal terminals, and the second end portion of the first IDT and the first end portion of the third IDT are connected to the ground potential, and thereby, substantially equal parasitic impedances are each added to the first and second balanced signal terminals.
In the third preferred embodiment of the present invention, the first end portion of the first IDT and the second end portion of the third IDT are each electrically connected to a first balanced signal terminal, and the second end portion of the first IDT and the first end portion of the third IDT are electrically connected to a second balanced signal terminal, and thereby substantially equal parasitic impedances are each added to the first and second balanced signal terminals.
In the surface acoustic wave filter in accordance with preferred embodiments of the present invention, it is preferable that the number of electrode fingers of the IDT electrically connected to the balanced signal terminal are an even number, and that the numbers of the electrode fingers electrically connected to the first and second balanced signal terminals connected to the IDT are equal, such that the degree of balance within the pass band is greatly improved, resulting in a greatly increased attenuation outside the pass band.
Furthermore, in the surface acoustic wave filter in accordance with preferred embodiments of the present invention, preferably, at least one IDT connected to a balanced signal terminal or an unbalanced signal terminal has a plurality of IDT portions divided in the direction that is substantially perpendicular to the propagation direction of a surface acoustic wave. In the IDT having the plurality of IDT portions divided in the direction that is substantially perpendicular to the propagation direction of a surface acoustic wave, since the impedance thereof is increased, a surface acoustic wave filter different input and output impedances is produced.
Moreover, in the surface acoustic wave filter in accordance with preferred embodiments of the present invention, preferably, at least one surface acoustic wave resonator connected to the above-described surface acoustic wave filter in series and/or parallel, is further included. Connecting at least one surface acoustic wave resonator to the surface acoustic wave filter in accordance with preferred embodiments of the present invention in series and/or parallel greatly enhances the attenuation in the vicinity of the pass bands.
The surface acoustic wave filter in accordance with preferred embodiments of the present invention is suitably used for, e.g., the band-pass filter for use in the RF stage of a portable telephone. Therefore, by using the surface acoustic wave filter in accordance with preferred embodiments of the present invention, a communication device provided with a band-pass filter which has a balanced-to-unbalanced conversion function and exhibits a large attenuation outside the pass band, and which is compact and superior in the frequency characteristic is provided.
The above and other elements, characteristics, features, and advantages of the present invention will become apparent from the following detailed description of preferred embodiments of the invention in conjunction with the accompanying drawings. Meanwhile, among these drawings, the drawings which show an electrode structure are depicted as having a fewer number of electrode fingers than in reality in order to simplify the drawings.